|The folders look like part of a content management system as do the files - admin.php; cms_style.css; index.php; template_e019.html. |
The error log and .htaccess files may also be part of the cms or they may be required by your host.
Since you don't really know, I'd recommend creating a folder at the root level (same level as index.php) and then move everything except the .htaccess and error log files into that folder keeping the same data structure.
You should then be able to create your own index.php file. If you don't have any problems with your hosted page(s), then you can *probably* assume that you're ready to do what you wish. If you have problems, then revert back to the old structure.
Either way, the index.php file is *likely* what is being served, so try editing it. You can comment out the current source by adding
Add something above and/or below the commented out section to test the new page; don't forget to save it as index.php.
You may also need to reset the permissions of index.php; if so, use chmod to reassign the file to whatever permissions it currently has.